1. Field of the Invention
The present invention relates to output buffer circuits including CMOSs for use in transfer circuits and the like for vehicles.
2. Description of the Background Art
Conventionally, an output buffer circuit generally comprises CMOSs in respect of a noise resistance problem. FIG. 1 is one example of a conventional output buffer circuit including two stages of inverters, the front inverter comprising a CMOS composed of a first MOSFET 1 of a P-channel type and a second MOSFET 2 of an N-channel type, the rear inverter comprising a CMOS composed of a third MOSFET 3 of a P-channel type and a fourth MOSFET 4 of an N-channel type. The source electrode of the first MOSFET 1 is connected to a positive power source V.sub.DD, and the source electrode of the second MOSFET 2 is connected to a negative power source V.sub.SS. The gate electrodes of the first and second MOSFETs 1 and 2 are connected together to an input terminal 5. The source electrode of the third MOSFET 3 is connected to the positive power source V.sub.DD, and the source electrode of the fourth MOSFET 4 is connected to the negative power source V.sub.SS. The gate electrodes of the third and fourth MOSFETs 3 and 4 are connected together to the drain electrodes of the first and second MOSFETs 1 and 2, and the drain electrodes of the third and fourth MOSFETs 1 and 2 are connected together to an output terminal 6 to make an inverter. When a signal is fed to the input terminal 5, the signal is inverted in the inverter and an inverted signal, as shown by a broken line in FIG. 2, is output from the output terminal 6. The waveform of the inverted output signal is affected by overshoot and impedance of electric transfer wires and thus is not entirely ideal.
When a conventional output buffer circuit of this kind is used for an output part of a pulse transfer part, a switching action often causes electromagnetic wave noise. When, in addition to the switching action, an overshoot, as shown in FIG. 2, is given to the signal, a high-frequency electromagnetic wave noise is also caused. When the output buffer circuit is employed in a transfer circuit for a multichannel communication or the like for a vehicle, the caused electromagnetic wave noise such as a radio wave noise cause trouble in a radio receiver in a vehicle. Hence, at least the radio wave band noise component in the electromagnetic wave noise generated by the output buffer circuit equipped in the pulse transfer circuit within the vehicle must be removed.
In case of an output buffer circuit used for a bus line driver within a unit, when there is a leakage path from the unit, a noise in a radio receiver is caused in the similar manner as described above.
A variety of countermeasures has been proposed for preventing this trouble caused by the electromagnetic wave noise output from the output buffer circuit.
Firstly, an RC filter is provided in the output side of the rear converter of the output buffer circuit shown in FIG. 1. That is, as shown in FIG. 3, the RC filter comprising a combination of a resister 7 and a capacitor 8 is arranged between the output side of the rear inverter and the output terminal 4. In this case, the high-frequency noise component can be decreased or removed and the leading edge of the waveform of the output signal is moderated, as shown by a broken line in FIG. 4.
Secondly, an integrator circuit is inserted in the output side of the output buffer circuit, as disclosed in Japanese Patent Laid-Open Specification No. 61-152125. As shown in FIG. 5, an integrator circuit comprises an operational amplifier 9, a resister 11, a capacitor 12, a negative input terminal 13 and a positive input terminal 14. The output signal of the output buffer circuit (not shown) is input to the negative input electrode of the operational amplifier 9 through the resister 11, and the negative input electrode and the output electrode of the operational amplifier 9 are coupled to each other through the capacitor 12. The output electrode of the operational amplifier 9 is linked to the output terminal 6. In this case, the rising and trailing timings of the output signal can be controlled, as shown by broken lines in FIG. 6.
Thirdly, as shown in FIG. 7, a Miller capacitor 15 is arranged between the input and output sides of the rear converter, that is, between the gate electrodes and the drain electrodes of the third and fourth MOSFETs 3 and 4, as disclosed in Japanese Patent Laid-Open Specification No. 61-152125.
However, in the first case, the inclinations of the leading and trailing edges of the output signal are the largest at the right after the leading and trailing start times, as shown in FIG. 4. Hence, when the resister 7 and the capacitor 8 are so determined that the inclinations of the largest inclined portions are moderated, the other moderate portions of the leading and trailing edges of the output signal are further moderated, resulting in that the pulse waveform of the entire signal is largely deformed. As a result, it becomes liable to receive the influence of other noises such as the ground noise, and the advantages of the digital communication will be reduced. Also, since the deformation of the pulse waveform also depends on the floating capacity in the output side, it is difficult to control exactly the slants at the rising and trailing of the signal. Hence, it is impossible to exactly prevent the generation of the electromagnetic wave noise in the certain frequency band such as the radio wave band.
In the second case, as shown in FIG. 6, although the inclinations at the rising and trailing of the signal can be exactly controlled, the rising and trailing start and end portions incline more steeply to cause the high-frequency noise component. In the third case, as shown in FIG. 7, since the Miller capacitor 15 is inserted between the input and output sides of the rear inverter, the AC component in the data signal input to the rear inverter always passes through the Miller capacitor 15 to the output terminal 6. As a result, the use of the pulse transfer circuit providing with the output buffer circuit may be restricted.